1. Field of the Invention
The invention relates to engine control for a crankcase scavenged, two-stroke engine, and more particularly to a system for reducing the exhaust gas hydrocarbons emitted from such an engine at and slightly above idle speed and low power requirements, by controlling the quantity of intake air and fuel delivered to the engine.
2. Description of the Relevant Art
In conventional four-stroke engines, as operator demand for engine power is increased from idle, the amount of air per cylinder supplied to the engine is typically increased. As air per cylinder is increased, the quantity of fuel delivered per cylinder is also increased, thereby maintaining the desired air-fuel ratio to achieve the desired engine performance and emission objectives.
The structure and operation of crankcase scavenged, two-stroke engines differ in many respects from that of conventional four-stroke engines. One difference concerns the manner in which fresh air is inducted, and burned fuel is exhausted by the engines. Conventional four-stroke engines have intake and exhaust valves within the cylinders to accomplish these tasks. Crankcase scavenged, two-stroke engines do not employ intake and exhaust valves but rather, intake and exhaust ports which open directly through the walls of the engine cylinders. As combustion is initiated, the piston moves in its downstroke within a cylinder, uncovering the exhaust port for release of the burned fuel, and shortly thereafter, uncovering the intake port to enable the entry of a fresh air charge, and assist in expulsion of the combustion components of the burned fuel.
A problem associated with crankcase scavenged, two-stroke engines has been the high level of hydrocarbons present in the exhaust gas. At speeds near engine idle, with light operator induced loading, the level of exhaust gas hydrocarbons is highly dependent upon the amount of air per cylinder delivered to the engine. This relationship is thought to result from the absence of valves in the two-stroke engine, and the near simultaneous opening of the exhaust and intake ports during the engine operating cycle. Presumably, an excessive quantity of air flowing through the intake port forces an amount of unburned fuel out of the exhaust port thereby increasing the hydrocarbon content of the exhaust gas.
If the conventional practice of increasing the mass air per cylinder flowing to the engine is followed in controlling the near idle operation of a crankcase scavenged, two-stroke engine upon operator demand for output power, the level of hydrocarbons in the engine exhaust may be unreasonably high. Consequently, a need exists for an alternative engine control for such engines operating at speeds near idle, with light operator induced loading.